The turbine of gas turbine engines such as those used in jet aircraft provides the power necessary to drive the compressor and accessories, and, in engines which do not make use solely of a jet for propulsion, the turbine provides the power to drive the shaft of a propeller or rotor. Energy produced from the continuous flow of hot gases released by the combustion system of the engine is extracted by the turbine which expands the gases to lower pressure and temperature. In order to produce the driving torque required in the gas turbine engine, turbines normally consist of one or more stages. Each stage of the turbine normally employs one row of stationary nozzle guide vanes fixedly mounted to the turbine case, and a rotor which includes a row of rotor blades circumferentially mounted to the rotor disk. The rotor disk is either formed integrally with or has a shaft flange which is bolted to the shaft of the turbine.
The blades in the rotor of the turbine each have a blade root, e.g., a dovetail root, adapted to mount within mating, circumferentially spaced slots formed around the rim of the rotor disk. The blades also include an air foil extending radially outwardly from the blade root which terminates at a blade tip. In view of the high rotational speeds of the turbine rotor blades and the mass of the materials which form the blades, proper balancing of the rotors of the turbine is extremely important. Any unbalance can seriously affect the rotating assembly bearings and engine operation.
One prior art method of balancing the rotor in the turbine of gas turbine engines has been to employ weights which are bolted to the disk shaft flange of the rotor disk at one or more locations about its circumference. Each weight produces a moment about the center of rotation of the rotor disk which is the product of the mass of the weight and its distance from the center of rotation. The number, position and mass of the weights required is determined first by weighing each individual rotor blade and categorizing them, and then balance testing the turbine rotor. After finding the unbalance of the turbine rotor, the results are compared with the weight of the blades. Each blade is installed in a selected dovetail slot of the rotor disk and final balancing of the rotor is obtained by mounting the balance weights to the disk shaft flanges.
One problem with this balancing method is that a relatively large amount of weight is often needed on the disk shaft flanges of the rotor in order to balance the rotor. This is because the radius or moment arm between the center of rotation of the rotor disk and its aft flange is small. In order to increase the moment produced by the weight over such a short moment arm, the magnitude of the weight must be substantial.
Another problem with mounting balance weights to the aft flange of the rotor disk is that the relatively small radius between the center of rotation of the rotor disk and its aft flange makes it difficult to accurately mount the weights on the aft flange at the desired angular position relative to the center of rotation of the rotor disk. This is particularly true for relatively small angular adjustments, e.g., 1.degree. or 2.degree., wherein the weights can be moved only a very small distance along the circumference of the aft flange to produce the desired angular adjustment relative to the center of rotation of the rotor disk.
In order to lessen the amount of weight required to balance the rotor, another approach in the prior art to balance turbine rotors involves mounting weights to the blade retainers which are located at the rim of the turbine rotor to prevent fore and aft movement of the rotor blades relative to the rotor disk. These blade retainers are located at a much higher radius from the center of rotation of the rotor disk and thus the moment arm between the center of rotation of the rotor disk and the weights is much larger. As a result, the magnitude of the weight can be reduced compared to the prior art method wherein the weights are mounted to the low radius, disk shaft flanges of the rotor disk.
The problem with mounting prior art balance weights to the blade retainers is that bolts and nuts are required to form the connection therebetween. More current turbine rotors have eliminated bolts and nuts for attaching blade retainers or seals to the rotor disk, and have replaced them with boltless blade retainers and seals. One advantage of boltless blade retainers is that stress concentrations are eliminated because bolt holes are no longer formed in the rotor disk or retainers. In addition, windage problems i.e., interference in the air flow around the turbine rotor created by the presence of obstruction such as bolts and nuts, are reduced by the use of boltless blade retainers and seals. As a result of these improvements in methods of attaching blade retainers and seals, no structure is provided for bolting prior art balance weights at the rim of the rotor disk.
This problem has been overcome to some extent by another prior art method of rotor balancing in which the rotor disk is formed with an arm mounted to the rotor disk at a location intermediate its center of rotation and rim. This arm supports an annular flange formed with circumferentially spaced holes. Balance weights are inserted through such holes and secured to the flange by pins or rivets to balance the rotor.
The problem with this design is that the arm and flange add additional weight and cost to the rotor. Moreover, the holes in the flange create stress concentrations which are likely to reduce the cyclic life of the rotor. Additionally, the protruding balance weights, arm and annular flange produce windage affects which could result in engine performance penalties.